Environmental Engineering Reference
In-Depth Information
variability in the cellular composition (Wetzel 2001). While pigments such as chlorophyll also vary, they
remain a commonly used metric for biomass as well as the trophic status of lakes and reservoirs.
16.1.1.3 Methods for Measuring Primary Production
Primary production is commonly estimated by the rates of change (e.g., rate of change in mass/
time). For example, with reference to Equation 16.1, the rate can be estimated by either the rates of
change of biomass, the consumption of carbon dioxide or nutrients, the production of oxygen, or
other means. The method used is often speciic to the types of autotrophs being sampled, such as
phytoplankton, periphyton, and macrophytes.
One example is using changes in oxygen concentrations. For lakes and reservoirs, one method
that has received wide attention is the light and dark bottle method (Wetzel 2001), which is used to
estimate the rate of primary production by the rate of change in oxygen concentrations. Essentially,
two standardized bottles are placed in a water column, where one can receive light (the light bottle)
and the other cannot (the dark bottle). The oxygen concentrations are measured initially and then the
bottles are incubated at speciic depths in the water column. The change in oxygen concentrations is
recorded. In the light bottle, both respiration and photosynthesis take place; so the change in oxygen
is an indication of GPP. In the dark bottle, only respiration (R) takes place. So, the difference in oxy-
gen concentrations between the two bottles relects the net primary production (NPP = GPP - R).
Similarly, changes in carbon dioxide are used as indicators of production. However, the use of
carbon dioxide is complicated by interactions between species of inorganic carbon, which provide a
buffering capacity as indicated by their alkalinity. Alternatively, measurements of the rate of incor-
poration of the
14
C tracer into organic matter have been used as a measure of the rate of primary
production (Wetzel 2001). The labeled carbon is added to water (such as in a bottle or an enclosure)
and the quantity of organic material (e.g., of phytopklankton) is measured after incubation, provid-
ing a direct measure of the rate of production. The
14
C method is much more sensitive (by three
orders of magnitude) than the light/dark bottle oxygen method (Wetzel 2001).
For periphyton and macrophytes, changes in the accrual of biomass may be measured. Other
methods include changes in oxygen or carbon concentrations using methods similar to those used for
phytoplankton, but in other types of enclosures. For example, apical portions of macrophyte shoots
may be incubated in lasks. For
in situ
measurements of macrophyte productivity, Vollenweider
(1969) recommends using clear Plexiglas cylinders.
16.1.2 S
econdary
p
roductIon
Secondary production refers to the production by the consumers: both invertebrates and vertebrates.
Secondary consumption is much more dificult to estimate accurately than is primary produc-
tion (Wetzel 2001). The reasons include the following: trophic interactions are complex, the sizes
between organisms and life stages vary greatly, organisms vary in the number of generations per
year (voitinism), and they are mobile. For example, populations that are multivoltine have higher
rates of production than those that are univoltine (Downing 1984).
Secondary production is based on changes in the number of animals, biomass, and growth rates.
The rates may be for different types or communities of heterotrophs, such as the bacteria, zooplank-
ton, and benthic communities or ish and may vary with time. For example, secondary production
rates for benthic invertebrates are:
•
Often lowest during summer
•
Generally greater in running than standing water
•
At least ive to ten times greater for nonpredatory benthic organisms than for predacious
benthic organisms
•
Two to ive times higher than the zooplankton in the zoobenthos of shallow lakes with low
mean depths
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